New York Mayor Bill de Blasio visits the Inner Force Early Tots Childhood Learning Center, a community-based organization that’s part of the pre-K program, in the Brooklyn borough of New York on the first day of the mayor’s expansion of early childhood education, in 2014. Free school for 4-year-olds was de Blasio’s signature issue during his successful run for mayor in 2013. Facing re-election he has announced an ambitious plan to expand the effort to 3-year-olds.

NEW YORK — Free preschool for 4-year-olds was a signature issue that helped New York City’s mayor win office in 2013.

Now up for re-election, Bill de Blasio wants to add thousands of classroom seats for 3-year-olds, too.

His initiative, which could eventually enroll up to 62,000 kids, would be among the largest public investments in preschool in the United States for children that young.

A few states, including Florida and Oklahoma, have universal, publicly funded pre-K for 4-year-olds, but such programs for 3-year-olds are much more rare.

Education advocates said the plan announced last week by de Blasio, a Democrat, could be a national model if it succeeds.

“The kind of research it’s going to generate, as well as student outcomes, are going to show that it’s possible at scale to build a public system for preschool, and that’s never been done,” said Shael Polakow-Suransky, president of Bank Street College of Education, who served as deputy New York City schools chancellor under de Blasio’s predecessor, Mayor Michael Bloomberg.

A successful rollout could also help de Blasio raise his national political profile, something he aspired to do in his first term but couldn’t quite achieve as two other New Yorkers — Donald Trump and Hillary Clinton — took center stage.

New York City rolled out universal prekindergarten for 4-year-olds over just two years after de Blasio took office. The program now serves nearly 70,000 children in a public school district with 1.1 million pupils.

Under de Blasio’s plan, free preschool for 3-year-olds will start this fall in two low-income neighborhoods in Brooklyn and the Bronx, and then expand citywide by 2021.

The mayor said research shows that children who are enrolled in high-quality preschool at age 3 grow up to earn higher salaries and enjoy better health than their peers who didn’t attend preschool or who attended lower-quality programs. It can also reduce the burden on working parents who would otherwise have to pay for costly childcare.

“We know there’s a precious opportunity to reach children at the moment when they can learn and grow the best,” de Blasio said. “Bluntly, in a society where people are working longer and longer hours and are more stressed in so many ways, it’s going to be something a lot of parents think is practically really important for their lives.”

Some critics of de Blasio’s education policies, including backers of charter schools and private-school vouchers, questioned whether the New York City public school system will provide the kind of high-quality preschool that helps children thrive.

“Mayor de Blasio has a terrible record running K-12 and to distract voters, he’s turning attention to a new program,” said Jenny Sedlis, executive director of the pro-charter group StudentsFirstNY. “It’s a cynical election year play that will do nothing to help the kids stuck in failing schools now.”

Publicly funded preschool for 3-year-olds is the norm in much of the developed world — France has had it since 1980 — but it is rare in the United States.

Washington, D.C. is one of the few American cities that currently offers free preschool for 3-year-olds, but the Washington program serves 5,700 children, fewer than one-tenth the number New York City is hoping to enroll.

Hirokazu Yoshikawa, a professor at New York University’s Steinhardt School of Culture, Education, and Human Development and a co-author of the book “Cradle to Kindergarten: A New Plan to Combat Inequality,” said public commitment to early childhood education has often been a tough sell in the United States.

“Economically, it actually makes sense, but it’s an up-front investment where some of the benefits don’t show up until years later,” Yoshikawa said.

Deputy New York City schools chancellor Josh Wallack said the preschool program for 3-year-olds will focus on social and emotional development as well as language skills that will lay the foundation for learning to read.

De Blasio said the city will spend $177 million annually on the program by 2021, but will need another $700 million from the state and federal governments to fully fund the classes.

Asked if it was realistic to count on state and federal funds, de Blasio said he did not want to be “held back by our current reality.”

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LONDON — Theresa May announced Friday that she will step down as U.K. Conservative Party leader on June 7, admitting defeat in her attempt to take Britain out of the European Union and sparking a contest to become the country’s next prime minister. She will stay as caretaker prime minister until the new leader is chosen, a process likely to...